pope benedict opened a 40-day period of humility. having sent his own humble and revolutionary message. no one isser replaceable, even a pope. >> translator: as i prepare to conclude, i ask for special remembrance in your prayer. >> reporter: the 85-year-old looked frail leading his final major mass. some recorded the moment on ipads, and the massive audience gave him a rare standing ovation. the pontiff looking moved. out side, we found the audience had been moved, as well. do you get the sense that people inside felt like they were saying goodbye to the pope? >> yes, very much so. i felt like that. when he left the altar and he was blessing everybody, we knew that would be for the last time. that makes you very emotional. >> this is a once-in-a-lifetime opportunity. how often can you say you went to the vatican for ash wednesday? >> reporter: earlier he explained why he retired, admitting the papacy is a job and he could no longer perform it. i did this in full liberty, he said in italian, for the good of the church. the first time